1. Field of the Invention
The present invention relates to an active type solid-state imaging device having a transistor, such as an MOSFET (metal-oxide-semiconductor type field effect transistor) or a junction gate FET, which accumulates signal charges generated by photoelectric conversion of incident light and outputs an electric signal corresponding to the accumulated signal charges, and a method for fabricating such an active type solid-state imaging device.
2. Description of the Related Art
A charge coupled device (CCD) type has been a mainstream of solid-state imaging apparatuses and widely utilized in various fields. In such a CCD type imaging apparatus, signal charges obtained by photoelectric conversion and accumulated using a photodiode or a MOS diode are transferred through a CCD transfer channel to a charge detection section with high sensitivity, where the signal charges are converted into a voltage signal. The CCD type imaging apparatus is therefore characterized in a high S/N ratio and high output voltage.
In the recent trend of reducing the size of the imaging apparatus and increasing the number of pixels, the size of each pixel has reduced thus decreasing the amount of charge which can be transferred by the CCD. This has caused a serious problem of lowering a dynamic range of the apparatus. Moreover, since the CCD as a whole is driven with several phases of clocks, the load capacity increases and high driving voltage is required. Therefore, as the number of pixels increases, the power consumption significantly increases.
In order to overcome the above problems, an active type imaging apparatus has been proposed, where signal charges generated in respective pixels are not directly read out, but are read out by a scanning circuit after the signal charges are amplified in the respective pixels. Such an apparatus eliminates the limit on the signal magnitude and makes the dynamic range broader than that of the CCD type. Moreover, in the active type imaging device, since only a horizontal line and a perpendicular line corresponding to a pixel to be read out can be driven with a lower voltage, the power consumption is smaller than that required for the CCD type.
A transistor is generally used for amplifying signal charges in each pixel of such an active type imaging apparatus. Transistors are classified into a SIT (static induction transistor) type, a bipolar type, a FET type (a MOS type and a junction type), and the like. Among these types of transistors, a MOSFET is advantageously used for this purpose in consideration of the configuration of the entire apparatus, because a scanning circuit for signal readout can be constructed more easily using this type of transistor. In other words, while a SIT type or a bipolar type transistor is formed in a depth direction, a MOSFET type transistor is formed in a plane direction. The latter is more advantageous in the fabrication of the apparatus. Especially, an imaging apparatus in which only a single MOSFET is included in each pixel is more advantageous to increase the pixel density. As this type of active type imaging apparatus, a type proposed in Japanese Laid-Open Patent Publication No. 8-78653 (hereinbelow, this type is referred to as a TGMIS (twin gate MOS image sensor) since two gate electrodes are used), a CMD (charge modulation device) type, and the like have been reported.
FIG. 12 is a sectional view of a pixel portion of a TGMIS active type solid-state imaging apparatus proposed in the above-mentioned Japanese Laid-Open Patent Publication No. 8-78653.
Referring to FIG. 12, a second gate electrode 3 is formed on a p-type semiconductor substrate 1 via an insulating film 2. Then, an n.sup.- -type well region 4 is formed in the surface portion of the semiconductor substrate 1, and an n.sup.+ -type diffusion region 5 for a source and an n.sup.+ -type diffusion region 6 for a drain are formed in the n.sup.- -type well region 4. A first gate electrode 7 is formed on the n.sup.- -type well region 4 via the insulating film 2. The first gate electrode 7 as a gate, and the n.sup.+ -type diffusion regions 5 and 6 as a source and a drain, respectively, constitute a MOS transistor. Thus, a TGMIS active type imaging device is constructed. A plurality of such imaging devices are arranged in a matrix as pixels to complete a TGMIS active type imaging apparatus.
With the above configuration, pairs of electrons and holes are generated in the surface portion of the n.sup.- -type well region 4 by photoelectric conversion of light energy h.nu. incident through the first gate electrode 7. Electrons flow to the source and drain regions, while holes are accumulated in the n.sup.- -type well layer 4 at the interface with the insulating film 2 as signal charges. In this way, the MOS transistor formed in each pixel allows signal charges to be generated by photoelectric conversion of incident light and accumulated. An electric signal corresponding to the accumulated signal charges is then output.
FIG. 13 is a sectional view of a pixel portion of a conventional CMD active type solid-state imaging apparatus. An n.sup.- -type well layer 12 is formed on a p-type semiconductor substrate 11 in a buried-in fashion. A gate electrode 14 is formed on the n.sup.- -type well layer 12 via an insulating film 13. A source region 15 and a drain region 16 which are high-concentration n.sup.+ -type regions are formed in the surf ace portion of the n.sup.- -type well layer 12 so that they are separated by the gate electrode 14.
However, the above-mentioned conventional imaging apparatuses have the following disadvantages. In the TGMIS active type imaging apparatus shown in FIG. 12, since the source and drain regions 5 and 6 are high-concentration n.sup.+ -type diffusion regions, a large concentration gradient exists between the source and drain regions 5 and 6 and the low-concentration n.sup.- -type well region 4. This is considered to strengthen the electric field in the surface portion of the regions and thus accelerate the movement of carriers, generating impact ions. As a result, pairs of electrons and holes are generated in the surface portion of the n.sup.- -type well region 4 under the first gate electrode 7 where photoelectric conversion occurs. Electrons of these pairs flow to the source and drain regions, while dark current is generated by the remaining holes. These holes also accumulate at the interface with the insulating film 2 as additional signal charges.
In the CMD active type solid-state imaging apparatus shown in FIG. 13, also, the source and drain regions 15 and 16 are high-concentration n.sup.+ -type regions. Therefore, the same drawbacks associated with the TGMIS active type imaging apparatus occurs.
Thus, the conventional active type solid-state imaging apparatus includes a first-conductivity type substrate, a low-concentration second-conductivity type well region formed on the substrate as a channel, a gate electrode formed on the well region via an insulating film, and high-concentration second-conductivity type source and drain regions formed in the surface portion of the well region. The ion concentration gradient is sharp at and around the interfaces between the drain and source regions and a photoelectric conversion portion of the well region. This causes pseudo signal charges to be generated by impact ions, increasing dark current in the photoelectric conversion portion.
Therefore, there is a strong need in the art for an imaging device which reduces dark current in a photoelectric conversion portion by minimizing the generation of impact ions.